Thermionic valve cathode



June 22, 1965 F. w. R. FARRow 3,191,089

THERMIONIG VALVE CATHODE Filed Aug. 29, 1960 I NVENTR United States Patent O 3,191,089 THERMIONIC VALVE CATHODE` Frederic William Rodbard Farrow, Enfield, England, as-

signor to Associated Electrical Industries Limited, London, England, a British company Filed Ang. 29, 1960, Ser. No. 52,476 Claims priority, application Great Britain, Sept. 3, 1959, 30,155/ 59 2 Claims. (Cl. 313-270) This invent-ion relates to thermionic valve cathodes of the indirectly heated type.

Such cathodes are heated by means of separate heaters which are required to be in proximity to the cathode for good heat transfer, but electrically insulated therefrom. These requirements are to some extent conicting and it is an object of the present invention to provide a structure which satisfies both requirements.

According to the present invention an indirectly heated cathode structure comprises a sleeve having an external electron ernissive surface, a tensioned wire provided with an insulating coating extending Within the sleeve, a heater Wire Wound helically round said coating, and electrical connections to each end of the heater Wire and to the sleeve. The tensioned Wire may be of linear form and extend centrally Within the sleeve, or may be of hairpin form, both limbs being located Within the sleeve.

Preferably one end of the heater Wire is electrically connected to the tensioned Wire which thus forms part of the heater current supply circuit and the ow of heater current through the tensioned wire generates heat which assists in heating the cathode. With this arrangement the two external connections to the heater wire can both be brought out at one end of the cathode structure.

The heater Wire may be in the form of a coiled-coil.

The tension in the Wire can be maintained by a spring secured thereto and anchored to an insulating disc support of the assembly. Conveniently the spring and the heater Wire are both secured to the end of the tensioned wire opposite to the external connections. In one embodiment of the invention the tensioned Wire is provided with a nickel sleeve at that end to facilitate Welding the spring and the heater thereto.

In order that the invention may be more fully understood reference will now be made to the accompanying drawing, the single figure of which shows part of a thermionic valve embodying the invention.

Referring to the drawing a cathode structure 1 comprises a sleeve cathode 2 having a coating which is electron emissive when heated. Heat is supplied by means of a separate heater assembly extending axially within and spaced from the sleeve. This heater assembly comprises a central wire 3 having an insulating coating 4 applied thereto over the major part of its length. Coating 4 may consist of alumina and be applied by cataphoresis. Wire 3 is secured at one end 5 to a rigid external electrical connection 6 of the valve and at the other end 7 to a cantilever spring 3 which in turn is anchored at 9 to a mica support disc 10 supporting the cathode structure. Spring 8 is flexed so that wire 3 -is in tension and while illustrated as a cantilever spring, other forms of spring such as a helical spring can equally Well be used. Another mica support disc 11 is also provided at the other end lCe of the cathode structure` in conventional manner. A helical heater Wire 12 extends round insulating coating 4 and is secured at one of its ends 13 to an external electrical connection 14 of the valve and at its other end 15 to the end 7 of tensioned wire 3.

Heater wire 12 preferably takes the form of a coiledcoil, and may be coated with insulation, c g. alumina, as is the Wire 3. One method of constructing the coiledcoil is to wind a heater Wire in helical fashion onto a wire mandrel which can, for instance, be of molybdenum, which helix, still containing its mandrel, is then helically Wound round insulating coating 4. Alternatively the helix can be wound in helical fashion on to a removable mandrel Which after removal is replaced by the coated wire. In either case the original wire mandrel used to wind the rst helix is dissolved out with the coiled-coil heater wire in its final position. Insulation, if desired, is then applied, for example by cataphoresis.

The initial heating time of the cathode assembly depends, in part, on the thermal capacity of the insulating coating on the tensioned wire 3. Accordingly if the thickness of the coating is decreased, the heating time will be decreased, and vice versa. If the heater wire 12 is coated with insulation, this will increase the heating time of the cathode, which may be desirable in some cases. An insulating coating on the heater wire reduces thermionic emission therefrom.

Current for heating the cathode structure is supplied by applying a potential diierence between connections 6 and 14. The path of the heater current thus extends from connection 6 through the length of the tensioned wire 3 to one end of the heater Wire and thence through heater wire 12 to the other external connection 14. The heating up time of the cathode is thus reduced by the fact that the tensioned wire is heated by the passage of the heater current.

By maintaining the heater and its support wire 3 in tension its alignment relative to sleeve 2 is maintained with changes in temperature and thus the heater Wire can be held in spaced relationship to sleeve 2 Without the need of interposing any insulating spacers which restrict heat flow. The absence of such spacers enables the initial heating time of the cathode to be shortened and results in the highest possible value of insulation between heater Wire and cathode, namely the insulation of a vacuum.

To facilitate the welding of spring 8 and heater wire 12 to the end 7 of tensioned wire 3 a thin nickel tube 16 is slipped over Wire 3 and is secured to this wire and spring 8 and heater Wire 12.

The cathode sleeve 2 is provided with an external connection shown at 15.

As mentioned hereinbefore, the wire 3 may be of hairpin form and be tensioned by a spring, such as cantilever spring 8 engaging the bight of the hairpin. Each limb of the hairpin will then be arranged to support a section of the heater Wire. Heating current will also be passed through the hairpin-shaped thermionic wire in this` embodiment.

When a helical spring is used in place of the cantilever spring 8, the anchoring arrangement requires modification. Conveniently an additional mica may be positioned above the cathode tube as a platform which would have a hole in the centre through which the tensioned W-ire 3 would pass. Thereafter the Wire would pass up through 4 the centre of the spring and be secured to the top of it.

What I claim is:

1. An indirectly heated cathode structure comprising a cathode sleeve having an external electron emissive surface, a linear tensioned support wire extending axially Within said sleeve, an insulating coating for said support wire, a doubly helical heater Winding formed of a helically coiled Wire helically Wrapped around and in mechanical Contact with Said support Wire coating, an electrical connection between one end of said heater wire and one rend of said support wire, external electrical connections for heater current to the other end of said support Wire land the other end of said heater Wire, and an external electrical connection to said cathode sleeve.

2. An indirectly heated cathode structure comprising a cathode sleeve having an external electron emissive surface, a tensioned support Wire of hairpin shape extending Within said cathode sleeve, tensioning means engaging the bight of said hairpin-shaped support wire, an insulating coating for said support Wire, a doubly helical heater l winding formed of a helically coiled Wire helically wrapped around and in mechanical contact with said support wire, external electrical connections for heater current to said support Wire and said heater Wire, and an external electrical connection to said cathode sleeve.

References Cited by the Examiner UNITED STATES PATENTS GEORGE N. WESTBY, Primary Examiner.

RALPH G. NILSON, BENNETT G. MELLER,

Y Examiners. 

1. AN INDIRECTLY HEATED CATHODE STRUCTURE COMPRISING A CATHODE SLEEVE HAVING AN EXTERNAL ELECTRON EMISSIVE SURFACE, A LINEAR TENSIONED SUPPORT WIRE EXTENDING AXIALLY WITHIN SAID SLEEVE, AN INSULATING COATING FOR SAID SUPPORT WIRE, A DOUBLY HELICAL WRAPPED AROUND AND IN MECHANICAL COILED WIRE HELICALLY WRAPPED AROUND AND IN MECHANICAL CONTACT WITH SAID SUPPORT WIRE COATING, AND ELECTRICAL CONNECTION BETWEEN ONE END OF SAID HEATER WIRE AND ONE END OF SAID SUPPORT WIRE, EXTERNAL ELECTRICAL CONNECTIONS FOR HEATER CURRENT TO THE OTHER END OF SAID SUPPORT WIRE AND THE OTHER END OF SAID HEATER WIRE, AND AN EXTERNAL ELECTRICAL CONNECTION OF SAID CATHODE SLEEVE. 